Multifunctional weak polyelectrolyte multilayers for membrane applications

Thin polymer coatings in the form of polyelectrolyte multilayers (PEM) are an interesting class of coatings for the surface modification of materials to provide functional interfaces. These layers are prepared by layer-by-layer (LbL) assembly, an attractive technique with control over the thickness and the properties of the multilayer. This LbL technique involves the buildup of PEM’s by alternate exposure of a charged substrate to solutions of polycations and polyanions, in combination with a rinsing step between each deposition step to remove weakly associated polymer chains. LbL is a versatile technique where control over multilayer properties such as thickness, charge and density, can be easily achieved simply by the choice of polyelectrolytes, the number of coating steps and the coating conditions (pH, ionic strength etc.). As the layers are very thin, usually in the order of nanometers, they need to be characterized precisely in detail on model surfaces prior to coating on the desired substrate. LbL is a suitable surface modification technique for any size and shape of the substrate, and if the substrate is porous such as an ultrafiltration (UF) membrane support, then the PEM can act as a membrane separation layer for nanofiltration (NF) or solvent resistant nanofiltration (SRNF) applications. Additionally these multilayers can be useful to alleviate the problem of fouling in membrane technology, as these multilayers can work as antifouling layers and can also make cleaning of fouled membrane easier by sacrificial removal with some trigger (e.g., salt, pH, surfactant etc.). To provide these functionalities to membranes one suitable choice of PEs is a pair of weak polyelectrolytes such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH). Since the charge density is not fixed in these weak polyelectrolytes, control over adsorption, desorption and separation performance of multilayers for membrane applications can be achieved by simple alterations of the pH of the coating solutions. In this thesis, the focus is to investigate the use of weak PEM’s on UF membrane supports to make antifouling and easy to clean hollow fiber NF membranes for micropollutants removal from water and for stable SRNF membranes.